Initial Report on the SIS Internal Background

--Keith C. Gendreau, MIT

Abstract

We present an initial report on the instrumental background of the Solid State Imaging
Spectrometers aboard ASCA. Factors which affect the intrumental background include the
local magnetic rigidity and CCD clocking mode. The spatial, spectral, and temporal
dependence of the background will also be addressed.

A Brief Description of the Intrumentation

The two Solid State Imaging Spectrometers (SIS) aboard ASCA are X-ray CCD cameras
made at MIT.
In order to understand some of the instrumental background features of the SIS, we review
here
a basic description of the detectors.

Each detector consists of a hybrid array of four-edge abuttable frame-store CCDs made at
MIT's
Lincoln Laboratory. Each CCD has 27x27 micron pixels in the imaging array and 18x24
micron pixels
in the framestore array. Both the imaging array and framestore array have 420x420 pixels.

Each CCD is approximately 380 microns thick, of which about 25-30 microns are depleted
for charge
detection. The charge cloud produced by an event occuring inside this front 25-30 micron
depletion
region will be completely collected and registered (usually) as a single pixel or two-pixel
event.
The remaining 350-355 microns of silicon will still participate in photoelectric and other
ionizing
events with X-rays, g-rays, and high energy particles, but the charge clouds produced will
expand
freely with time until either recombination occurs or the charge enters a depletion region.
Events
which interact deep from the front surface in the CCD tend to be higher energy photons or
particles. The expanding charge clouds from these deep background events can encompass
several pixels and thus tend to get graded as multiple pixel events (grades 6 and 7; see the
AO description of event grading or Ricker, et al. (1994)).

A factor which affects some of this grade rejection of the background is a feature on the
CCD called
the "Back Diode." The Back Diode is plate attached to the back side of the CCD which can
be set at
a high potential. The plate then can deplete the silicon reducing the 350-355 micron field-
free
region by about 200 microns. The effect is to reduce the maximum size that an individual
deep-interacting event can affect, thus reducing dead area. On ASCA, the back diode has
two states:
a high voltage and a low voltage state. These result is different back depletion region
depth, and
thus a possible effect on the grading of deep-interacting background events. Figure 1
shows a cross
section of a part of the CCD with the various depletion and field-free regions shown.

A more complete description of the CCDs can be found in Burke, et al. (1991).

The CCD is mounted on a ceramic plate inside of a gold coated kovar (iron, nickel, cobalt
alloy)
package. The package is mounted within a gold coated aluminum housing, which is
surrounded by a
polyethylene shield. Figure 2 shows a schematic of the arrangement. The polyethylene
shield and
aluminum housing provide protection against radiation damage in the CCD (Gendreau, et
al. (1993)).
The optical block filter shown in the figure above the CCD is extremely thin (1000 angstom
Lexan + 800 angstrom aluminum). A beryllium door is also near the CCD (but not in a
clear line of sight).
The door was closed for the first few days after the SIS turn on and is now permanently
open.

Figure 1: Cross section of CCD (not to scale) showing X-rays (X) illuminating the
CCD
from above. The X-rays pass through a thin deadlayer gate structure consisting of
polysilicon,
silicon oxide, and silicon nitride. X-rays which interact within the front side depletion
region
usually produce charge clouds which are entirely collected within 1 or 2 pixels (event "S").
Higher
energy photons and particles interact deeper in the silicon. Events which originate in the
field
free region (event "D") will produce expanding charge clouds which affect several pixels.
Particles
which interact even deeper in the "back diode depletion region" will be entirely collected by
the
back diode ("R") and not register as an event. All dimensions in the figure are approximate.

Figure 2: Cross section of CCD camera (not to scale) showing X-rays (X)
illuminating the
detector from above. The figure to the left is slice through the whole camera. A
polyethylene shield
(A) surrounds the gold plated aluminum housing (B) of the camera. Other material around
the CCD
include: a beryllium door (C) now permanently open, an aluminized lexan optical blocking
filter (F),
and a gold plated kovar block (D). The figure to the right is a blow-up of the cross section
of the
region around the CCDs. The CCDs sit on a ceramic plate inside of a gold plated kovar
package. A
gold-plated kovar framestore shield is mounted on the package allowing X-rays to
illuminate the
imaging arrays of the CCDs.

A Two Component Model for the Origin of the Instrumental Background and the
Effects of
CCD Operating Mode

The internal background of the SIS can be divided into two components:

internal background falling on the imaging array and

internal background falling on the framestore array.

The first component is proportional to the live time (the sky exposure time), while the
second component is proportional to the number of readouts. The number of readouts (Nr)
is proportional to the live time (Tl) by the following formula:

(1)

where the frame exposure time (Tm) is a function of clocking mode:

(2)

Thus if you calculate internal background rates based on sky exposure time, the
contribution of the internal background on the framestore array will be higher as you go
from 4- to 2- to 1-CCD mode.

In general, there will be a position dependence (in detector coordinates) to the internal
background. For the imaging array, it is not easy to see where the background is coming
from since the geometry is rather complicated. For the component due to the internal
background on the framestore array, the rate will increase with row number, since the
“exposure time” of a row in the framestore area increases with row number. Considering
this, below is an expression for the background counting rate per row (R(r)) read out:

(3)

where

D(r)

is the sky background (0 when looking at Dark Earth) in counts s-1 row-1,

Ii(r)

is the internal background per row on imaging array in counts s-1 row-1,

I0

is the intrinsic internal background rate per row on the framestore region in counts s-1
row-1,

Rrow

is the readout time per row (9.4 msec)

Pa

is the ratio of the pixel areas in the imaging and framestore region (= 0.59 )

Thus we should see a slope to the internal background with row.

When equation (3) is multiplied by the live time, Tl, then it is easy to see that the sky and
image
area components scale with time, and that (via equation (1)) the framestore component is
proportional
to the number of readouts.

If we integrate equation (3) over 420 rows and then divide by 420 rows, then we will get
the
counting rate per CCD per second (Cm).
We can use the last equation to determine the ratio of internal counting rates for different
CCD clocking
modes. This could also be used to predict total background rates (Sky+internal) for
different modes. For now, consider looking at the dark Earth, then we may approximate
that

(5)

If we make the additional assumption that

(6)

then equation (4) yields the following results for 1, 2, and 4-CCD mode operation:

(7)

In particular, the predicted ratio of internal background rates for both 4-CCD to 1-CCD
mode and
also 4-CCD to 2-CCD mode observations per CCD should be:

(8)

Data

Event Grade Rejection: We normally do science analysis with data of grade less
than 5. This
practice results in a rejection of about 98 percent of the all cosmic ray induced events. The
higher grade
events occur with a frequency of about 1 count s^(-1) cm^(-2) of detector. The two
component model applies
here, and thus the rate is clocking-mode dependent and row dependent. There is some
evidence that the
high grade event rate in the framestore array is larger than it is in the imaging array. This
may be
a result of both showering effects from the framestore shield as well as by grading
differences due
to the smaller pixels in the frame store array. Since we can reject the high grade events so
easily,
we will not discuss them further here.

Dark Earth Data Selection and Cleaning: To handle the remaining 2
percent of the internal background, 110 ksec of 4-CCD mode data were collected from
April to
September 1993 during satellite night while ASCA was looking at the dark side of the
Earth. We took
this "dark Earth" data only during satellite night to avoid optical light leakage problems.

The SIS is sensitive to optical light. Optical light contamination affects the SIS in two
ways:
it will lead to high grade events which often results in telemetry saturation (in particular
with
chips 2 and 3 of SIS-S0), and it will lead to gain offsets for normal X-rays. An additional
effect
that comes with optical light contamination in proximity to the bright Earth is the associated
atomic
oxygen K emission line (0.54 keV) from the bright Earth atmosphere. The complications
due to optical
contamination will be discussed elsewhere.

Hot and Flickering pixels were removed from the data by disregarding any pixels which
registered an
event twice. This is a reasonable thing to do considering that each CCD has over 160,000
pixels and
during the 110 ksec each accumulates about 1000 internal background events. If the high
frequency
flickering pixels (hot pixels) are left in, then several "lines" will appear in the resulting
pulse
height spectra. The inclusion of the very lowest frequency flickering pixels (e.g. including
pixels
which registered events 2 or 3 times during this 110 ksec) results in a steep power law
feature in the spectra. The flickering pixels will be discussed elsewhere in more detail.

Row Dependence: Equation (3) describes how the background counting rate should
vary with row
due to the extra exposure time in the framestore region. Figure 3 shows the variation in
grade 0-4
counts (in 0.4-12 keV) with row for the dark Earth data. The slope is consistent with what
would be
expected for 4-CCD mode operation with identical intrinsic rates over the imaging and
framestore
regions.

Spectral Features: The spectra (Figure 4) of the cleaned dark Earth data sets can be
characterized by a nearly flat continuum, fitted without using the telescope and detector
responses,
with fluorescence lines due to iron, nickel, aluminum, silicon, and gold. Table 1 lists the
line
features as well as the possible origin for most of the lines. Due to geometric considerations
(see Figure
2),
the origin of the iron and nickel emission lines is most likely the particle induced
fluorescence in the kovar framestore shield. The X-rays from framestore shield will be
landing
mostly on the framestore array of the CCD leading to strong spatial gradients and clocking
mode
effects by the two component model described above.

Figure
3:
The gradient in grade 0-4 events with row for the dark Earth data. The events represent
those from
pulse heights for 400 eV to 12 keV. All eight chips of the two SISs are combined here to
show the
effect. 4-CCD mode was employed.

Figure 4:
The spectrum grade 0-4 events for the dark Earth data. The events represent those from
pulse heights
for 400 eV to 10 keV. All 8 chips of the 2 SISs are shown. 4-CCD mode was employed.

There is also an oxygen line present, which we consider to be most likely of atmospheric
origin since
there is no Si-K line of comparable strength seen in the spectrum while the fluorescence
yield of
oxygen is more than an order of magnitude smaller than that of silicon and there is much
more silicon
around the sensitive part of the detector. The average spectrum of the darkEarth data set
from the
two SIS sensors has a total intensity of 7.3x10^(-4) ct s-1 keV-1 per CCD (6x10-4 ct s^(-
1) keV^(-1)
cm^(-2)) when in 4-CCD mode. More detailed rates will be given below.

Magnetic Rigidity Effects and Counting Rates: The background rates have a
dependence on the
local magnetic rigidity in which the satellite is located.
Figure
5
shows a histogram of fractional exposure as a function of magnetic rigidity. In this figure,
you
can see that for a typical observation, the local magnetic rigidity ranges from 4 to 15 GeV/C
with a
large hump peaked at about 12 GeV/C. To examine the rigidity effects on the background,
we have
divided the dark Earth data set into low (less than 12 GeV/C) and high (greater than 12
GeV/C)
subsets. Table 2 shows some of the count rates and model parameters found for these two
rigidity
ranges. The values quoted in table 2 are mean values for the listed sensor.

Figure 5: A histogram showing the fraction of an observation that is done while the
satellite
is within a region of given local magnetic rigidity. For the plot above, two histograms are
shown:
one for the collection of dark Earth data taken and one for the cosmic XRB data taken. The
similarity
of the curves helps to justify the use of the dark Earth data as background for the XRB
data.

The model parameters refer to "/b" models found in the XSPEC fitting package. We fit
power laws and
multiple Gaussian models to the data using the "/b" (background) option which allows us
to use the
detector response matrix without using the effective area information (i.e. we only use the
spectral
redistribution information and gain relation). For more information type help
background in XSPEC.

Table 2. Rigidity Dependencies and Sensor to Sensor Variations
There are a few things to notice in Table 2:

The total counting rate of SIS-S1 is 12 percent larger than that of SIS-S0.

The iron line intensity on S1 is about 5 times larger than it is on S0 while having very
similar
nickel intensities. This may indicate that the gold plating on the framestore shield of S0 is
thicker than it is for S1. The iron and nickel lines most likely originate in the kovar (an
Fe/Ni
alloy) framestore shield which is plated with a nominal thickness of 2 microns of gold.
The 1/e
length of a 6.4 keV photon in gold is about 1.5 microns, so the transmitted intensity would
vary
greatly with variations of the gold plating thickness.

The aluminum line intensity is about 2 times bigger on S0 than it is on S1. This may
be due to
variations in the thickness of gold plating on the aluminum camera bodies of the sensors.

The fluorescent line intensity is about 2 times bigger for the lower magnetic rigidity
data than
for the higher rigidity data. This may just reflect that the lines are a result of particle-
induced
fluorescence and the particle intensity increases as the local magnetic rigidity decreases.

For most of the analysis we have done, we choose data taken with magnetic rigidity greater
than 8
GeV/C. Table 3 shows the fit parameters for each of the individual CCDs with this rigidity
selection.
Again, the parameters apply to "/b" models found in XSPEC. Also, all lines are assumed to
have a
width (sigma) of 1eV. Line intensities are given in counts s-1CCD-1. We used the
XSPEC model
mo = powerlaw/b + gaussian/b + gaussian/b + gaussian/b + gaussian/b + gaussian/b +
gaussian/b.

Temporal Stability of the Instrumental Background: For a typical observation, the
local
magnetic rigidity defines the short term intensity of the instrumental background.
Although,
the number of flickering pixels may change suddenly during passages across the night/day
terminator
of the orbit as well as near the bright Earth. This will be discussed elsewhere.

To address the long-term stability, we have looked at the intensity of the 8-12 keV
spectrum from all
of the deep survey fields observed with ASCA. The instrumental background dominates
over the cosmic
X-ray background in this energy range (Gendreau, et al. (1994)). The observations
occurred at various
times from March to September 1994. No significant variations in the 8-12 keV flux was
measured.

Discussion

We have presented a two component model to the instrumental background on the SIS.
This model should
hold for any CCD used in frame transfer mode. Two important results of this model are
that the
measured instrumental background rates should change with clocking mode and that there
should be a
gradient in the instrumental background with row. This model can be exploited to try and
understand
the spectral differences of the instrumental background on the framestore array and imaging
array.
This type of analysis still needs to be done. A knowledge of the differences could be used
to
evaluate schemes for reducing the background by modifying the framestore array. Some
modifications
could be made with the SIS now on ASCA (at the cost of extra calibration), while others
would be
reserved for future missions with CCDs. Possible modifications include:

Changing the clock voltages on the framestore array to reduce the quantum efficiency
of the array
for particle induced events. This is currently possible with the SIS, but some recalibration
may be
necessary. This may also modify hot pixel (flickering pixel) growth rates.

Modification of split threshold level and grades in event detection. The pixels are
smaller and
shaped differently in the framestore region leading to differences in the grading of events.
This is
partly doable with the SIS, but would require a huge recalibration effort.

Modification of pixel shapes and sizes in the framestore array to enhance rejection by
grade of
particle induced events. This would be something to consider for future missions. A
possible
modification would be to make the framestore pixels really small so that even X-rays
produced by
particle interactions in materials near the framestore array appear as large events.

Modification of the framestore shield to reduce unwanted background features. Again,
this would
be something to consider for future missions. Possible modifications would be to increase
the gold
plating thickness so that the iron and nickel lines do not come through (if kovar is still
used).

Finally, we must consider where the instrumental background stands compared to the
cosmic X-ray
background. Figure 6 (Gendreau et al, 1994) shows the data from several deep field
observations made
during the ASCA PV phase. The models fitted to the dark Earth data of
Figure
4
are drawn into Figure 6. For energies less than about 5 keV, the sky background is more
important
than the instrumental background. Although, the aluminum K line at 1.5 keV might pose a
problem in
some cases.

For a future report, more dark Earth data will be collected in 1-, 2-, and 4-CCD mode.
This data may
come from the dark Earth crossings during the various AO cycles. Particular emphasis will
be made on
1- and 2-CCD mode data since the growth in the number of flickering pixels will be
causing telemetry
saturation problems for 4-CCD mode observations.

Figure 6: The spectrum of the Cosmic X-Ray background shown against the
instrumental background.
All data taken with magnetic rigidity greater than 8 GeV/C.